Creating Excel file with StringBuilder, running slow

Question

I have attached a file with the code I am talking about.  I am converting a program from VB.NET and I thought StringBuilder would be much more efficient.  I took the concatenation and changed it to StringBuilder, but it is running for ever.  Or is it because the insert is taking so long?  I commented out the concatenated code, just in case I needed it again.

This is taking too long for one file and I have to make multiple files.  The Excel file starts with a blank file with only a header row.

I hope someone can help me with this.

Thanks,
arep